PrepTest 123

[lcid:3590] Prep Test 123 LSAT — Logical Reasoning — S3 Logical reasoning

Question prompt

Feathers recently taken from Remaining source text redacted.
Why the credited answer is right

Credited answer: E

The notes below walk through why it fits the stem and how to eliminate the rest.

Argument or Facts

Argument

Valid or Flawed

Flawed

Question Type

Cause & Effect / Strengthen with Necessary Premise Questions

Stimulus Summary

Stuffed birds from the 1880s have less mercury than living birds. Mercury from fish accumulates in the birds. Therefore, fish have higher mercury levels today compared to the 1880s.

Answer Anticipation

The argument concludes that a phenomenon—different mercury levels in birds—has a particular explanation—different mercury levels in fish. Frequently, when you're tasked with finding a Necessary Assumption in an argument that concludes one explanation, the correct answer will eliminate an alternative explanation. This is because, in order for one explanation to be correct, all others have to be incorrect. Let's look for an answer choice that rules out another way that the modern birds could have heightened levels of mercury, or these old, stuffed birds could have less mercury. These alternative explanations, when dealing with two separate groups, often come from a difference between the two groups. Here, we're told one set of birds is stuffed and preserved, and the other is living, so answers might deal with the preservation process changing mercury levels.

Answer choices

  1. A
    the proportion of a Remaining source text redacted.
    Why choice A is not credited
    Incorrect. This answer choice doesn't rule out an alternative explanation—it provides one! If the older seabirds ate fewer fish, then their diet could explain the lower mercury levels even if fish had the same amount of mercury in them. This alternative explanation weakens the argument.
  2. B
    the amount of mercury Remaining source text redacted.
    Why choice B is not credited
    Incorrect. This answer is outside the scope of the argument. What matters is how much mercury is in the fish, not how it got there.
  3. C
    mercury derived from fish Remaining source text redacted.
    Why choice C is not credited
    Incorrect. The argument doesn't discuss whether the mercury levels are good or bad for the birds, just that it's different. This answer choice is out of scope.
  4. D
    the stuffed seabirds whose Remaining source text redacted.
    Why choice D is not credited
    Incorrect. First, the stimulus says mercury accumulates as the feathers grow, not as the bird gets older. It's possible that feathers are replaced so fast that it doesn't matter the bird's age. Second, if anything, this provides an alternative theory—the mercury levels weren't lower in fish, mercury was just given less time to accumulate in these younger birds. Third, the modern birds might also not have been fully grown, in which case the comparison was equivalent. That's a lot of reasons to get rid of this answer!
  5. E
    the process used to Remaining source text redacted.
    Why choice E matches the stem
    Correct. This answer choice eliminates an alternative explanation related to a key difference we learn about the two sets of birds. It's possible that the different mercury levels are a result of the preservation process. This answer rules out that possibility, which is necessary to justifying the explanation in the argument's conclusion.

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